Offshore drilling and production operations often utilize a column of pipes to facilitate the controlled routing of oil, gas, minerals or other deposits located subsea, to an offshore platform. The pipes may be riser pipes coupled serially end-to-end to form a riser string. The offshore platform may be a fixed platform (e.g., drilling or mining rig) or floating platform (e.g., marine vessel). The lower end of the riser string may be connected to a subsea drilling or production equipment, including a sea floor mining unit, or other associated subsea equipment, while the upper end of the riser string is supported by support structures located above sea level. Typically, a riser string is supported by a spider fitted into the opening of a rotary table installed on the offshore platform. The spider may include a slip bowl in which slips are peripherally distributed to surround the section (e.g., riser pipe) of the riser string to be gripped.
Riser strings deployed in subsea operations may include hundreds of vertical pipes serially connected by riser couplings (or riser joints), depending on the desired operating depth. The offshore operating environment inevitably exposes the riser string to extreme mechanical stresses. For example, riser strings supported by offshore floating platforms, including marine vessels, are exposed to cyclic bending stresses and torsional shock loading due to pitch and roll motions of the platform caused by strong wave and current forces during adverse weather or sea conditions. These undue mechanical stresses may occur during installation or operation of the riser string, and will invariably lead to stress-related material fatigue failure.
Therefore, it is desirable to provide support structures which can mitigate the cyclic stresses acting on the riser string during subsea operations to improve reliability and performance of the riser string.